Trunked RF repeater systems and digital trunked radio transceivers capable of handling communications between numerous mobile units and dispatcher consoles in a single area are known. Trunked RF repeater systems are used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users.
Briefly, each digitally trunked RF communication site is assigned a "control" RF channel and multiple "working" RF channels. The working channels carry actual communications traffic (e.g., analog FM, digitized voice, digital data, etc.). The control channel carries digital control signals between the repeater sites and user RF transceivers (radio) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available, the site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver and other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary single site trunked RF repeater system is disclosed in commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321 which are incorporated here by reference.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas, it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing RF communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site typically includes a site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another, a switching network, hereafter sometimes referred to as a "multisite switch", may be provided to establish audio and control signal pathways between repeaters of different sites. These pathways are set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in area A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the site's RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the site's control channel to the audio channel assigned to the call. However, this assigned working channel is applicable only within the area covered by that site.
In addition, the site controller sends the channel assignment to multisite switch (200) which assigns an internal audio time slot to the call. The multisite switch also sends a channel request over a control messaging bus to other site controllers having a designated callee within their site area. Audio signals are routed through the multisite switch such that audio pathways are created to serve one or more callees and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call. Each secondary working channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
Thus, the caller communicates with a radio unit or group of radio units in another area via the multisite switch. The call is initially transmitted to the primary site controller, routed through an assigned audio slot in the switch, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies multisite switch 200 that the call is terminated. The multisite switch propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio routing pathways.
Detailed description and operation of such a distributed multisite switch is set forth in commonly assigned U.S. Pat. No. 5,200,954 to Teel, Jr. et al which is also incorporated herein by reference.
Mobile radios often "roam" from one trunked RF communications site to another. As a radio roams, it may detect a deterioration in reception of the monitored control channel of its currently selected site. That deterioration may be measured using various techniques.
One technique measures received signal strength and is typically referred to as Received Signal Strength Indicator (RSSI). Although the RSSI adequately measures received signal strength, RSSI is not always a satisfactory measure of overall audio fidelity, i.e., the degree of exactness with which audio information is received by the radio which is based, of course, on the audio information that was transmitted. For example, a strong signal may nonetheless have low fidelity because of intermodulation products, co-channel interference, echoes, and other noise. Although it is certainly desirable for a roaming radio to switch from a currently selected site with weaker signal reception to an adjacent site with stronger signal reception, that switching decision should also take into account the fidelity of the communications channels of that adjacent site.
One way to address signal fidelity is with known coding techniques such as BCH and Hamming that detect and correct errors of one or more bits in a single data word and set a flag if excessive errors exist. Conventional CRC and bit counting techniques provide an indication of the receipt of a "good" or "not-good" message. One drawback of these approaches is that significant redundancy bits must be added to the transmitted bit stream to insure bit error detection/correction. These extra bits reduce the amount of actual message data that can be sent per frame. A reduction in data message throughput is obviously undesirable and unacceptable. In addition, other system constraints may prevent an increase in the data transmission baud rate to compensate for the reduced throughput. What is needed is an approach that takes advantage of existing redundancies in the data stream rather than adding new redundancies.
Another approach is to determine an overall message error rate (MER) of a communications channel which typically involves counting the number of detected or corrected bit errors. When the MER exceeds a certain threshold number, the radio initiates a scanning procedure to look for and lock onto other available control channels A drawback with the MER approach is that by the time the radio determines that the MER for the current channel exceeds a preset threshold, signal fidelity and strength have often significantly deteriorated and continue to deteriorate as the radio roams further away from the current site. In other words, when the threshold is exceeded, the current control channel signal strength is effectively "lost," and the radio is unable to communicate until another communications channel can be identified and engaged.
Furthermore, counting of the numbers of errors and comparing that count to a threshold is not a comprehensive and accurate approach to measuring signal fidelity. Consider for example the situation where three identically transmitted redundant messages A, B and C are received on a communications channel by a radio as follows:
______________________________________ Possible Inputs Message (T = True F = Flawed Bit) ______________________________________ A T T T F B T T F F C T F F F ______________________________________
Exclusive ORing (i.e. comparing) these messages results in the following:
______________________________________ XOR AB 0 0 1 0 XOR BC 0 1 0 0 XOR AC 0 1 1 0 ______________________________________
Just counting the errors by summing the number of "1's" after message comparison oversimplifies the signal fidelity analysis. There is no differentiation between one and two erroneous bits, the latter two bit errors obviously being more significant. Nor is there differentiation between all three bits being true and all three bits being flawed or in error. In that latter situation, the indication of signal fidelity is entirely false. Consequently, simple summation of bit errors fails to account for either the type or the degree of errors. A more comprehensive and higher resolution indicator of signal fidelity is needed.
The present invention overcomes the above-described problems providing a method and apparatus for determining with high resolution the fidelity of a communications channel without modifying (i.e. adding more redundancy bits) to the channel data stream as transmitted. Mobile/portable radios freely roam between multiple trunked RF communication sites and calculate the fidelity of communications received over current and alternate channels with comprehensive, high bit error resolution using a weighted average error dispersion procedure. This procedure generates for each group of redundant messages (and in some instances for subpreamble bits) from a preexisting messaging protocol format in the data stream received by a roaming radio in each message frame a weighted average error dispersion number whose incrementally increasing magnitude reflects deteriorating fidelity. Decisions about switching to alternate communications channels are made by the roaming radio based on this incrementally changing, weighted error dispersion number so that communication channels may be selected to achieve optimum signal fidelity and continuity of communication. In addition, an operator may programmably set for the radio a variable switching delta between error dispersion numbers of alternative communication channels relative to the current error dispersion number to ensure improved signal characteristics and communication continuity before initiating a change in communications channel.